T22: Collaboration

- presence: avatars ringed in each peer's color with where they are, press to
  follow; a breathing live dot; follow mode frames the view and stops on Escape
- remote cursors and selections painted over a field from its own layout
- comment threads with replies, reactions and resolve; a marker that opens a
  thread in a bubble; a sidebar of open and resolved threads
- annotations washed in their authors' colors, notes on hover
- suggestion mode: edits become suggestions word by word; track changes shows
  them struck and underlined, to accept or reject
- sharing: invite by email with checked chips, roles, the access list and who
  the link lets in, in one dialog
- primitives::hold_focus gives a mode its own focus, shared with ZenMode
- AvatarGroup's tuck and count shared; Avatar::ring_color; TagInput::check;
  TextInput::bounds_for_range; Popover::size
- icons: smile-plus, user-plus, message-square-diff
- gallery: the Collaboration page with scripted states

src/primitives/mod.rs

@@ -14,7 +14,7 @@ pub use backdrop::{Backdrop, Place};
 pub use disclosure::Disclosure;
 pub use divider::Divider;
 pub use focus::{FocusNext, FocusPrev, FocusRing, FocusScope};
-pub(crate) use focus::{Takeover, give_back, tab_stop, take_focus};
+pub(crate) use focus::{Takeover, give_back, hand_back, hold_focus, tab_stop, take_focus};
 pub use ghost::DragGhost;
 pub use icon::{Icon, IconName};
 pub use image::Image;